This application relates to output circuits associated with industrial controllers and more particularly to a method of preventing output circuits from producing false signals when power is first applied to those output circuits.
Industrial controllers such as those described in U.S. Pat. Nos. 3,810,118; 3,942,158; 4,165,534 and 4,442,504 are typically connected to industrial equipment used in assembly lines or to machine tools, to operate such lines or tools in accordance with a stored program. As distinct from conventional computer systems, industrial controllers generally physically control a process in "real time".
In industrial controllers, such as those disclosed in the above-cited patents, the stored program includes instructions which, when executed, examine the condition of selected inputs to the controller, from sensing devices on the controlled equipment, and energize or de-energize selected outputs from the controller to operate devices on the controlled equipment. Inputs to the controller may be discrete binary signals such as those from switches, which may detect limits of process variables such as motion, temperature, time, or other quantities, or the inputs may be analog measures of the process variables themselves, which are generally then converted to digital binary form for processing.
Likewise, the outputs of the controller may be either binary or "digital" outputs as implemented by mechanical or solid-state relays, or analog outputs produced by means of a digital to analog converter. Electrical isolation is ordinarily incorporated into the output circuits so that potentially harmful voltages and currents from the controlled process cannot damage the industrial controller or find a path through the industrial controller to other outputs. Isolation also serves to prevent undesired current paths in complex installations such as "ground loops", which may affect the integrity of the control system.
The isolation of the outputs is preserved by providing the outputs to the controller with a separate power supply. When power is first provided to the outputs of the industrial controller, the outputs may turn on momentarily despite the absence of an instruction to be in the "on" state from the stored program of the industrial controller. This momentary "on" state, upon application of power, is referred to as "flash-on" and typically lasts anywhere from a few microseconds to tens of milliseconds.
Flash-on is unnoticed in many industrial control applications where the output circuits control relatively slow mechanical actuators, such as contactors and motors, and where a short duration flash-on has no appreciable affect. Also, current practice is often to have the industrial controller and its outputs powered up at all times and restarted only under controlled conditions with collateral equipment deactivated. In such circumstances, flash-on will be unimportant.